Along with the drastic increase in communication demand in recent years, a multilevel phase modulation type optical communication system which encodes the phase of light and increases the capacity transmission by multi-valuing the phase of light is put to practical use. As phase modulators used in such a communication system, conventionally, Mach-Zehnder (hereinafter referred to as MZ) type modulators having an apparatus size of several tens mm or more using an electro-optical effect of a dielectric such as lithium niobate (LiNbO3) have been used. In addition, optical amplifiers such as erbium-doped fiber optical amplifiers are used to compensate for attenuation of the light intensity due to a loss caused by inserting a phase modulator in the optical path.
On the other hand, in response to demands for miniaturization of communication apparatuses, semiconductor phase modulators suitable for miniaturization are being actively developed. In the semiconductor phase modulator, for example, in Patent Document 1, an optical integrated device having insertion loss compensated by arranging a semiconductor phase modulator and a semiconductor optical amplifier (hereinafter referred to as SOA) on the same substrate in an integrated manner has been proposed.
Patent Document 1 describes a configuration in which an SOA is arranged in the post stage of the semiconductor MZ type phase modulator to attenuate the incident light due to the insertion loss of the semiconductor MZ type phase modulator. With this configuration, entering strong signal light into SOA is prevented so as to control the gain saturation of SOA. Applications of the SOA include applications for amplifying the intensity of continuous light and applications for amplifying modulated signal light with low distortion, but in any case, since the characteristics may be impaired by gain saturation, the method for controlling gain saturation of the SOA has been developed.
For example, as a method for controlling gain saturation as the SOA alone, for example, Patent Document 2 proposes a configuration in which changing the thickness of the optical guide layer provided adjacent to the active layer of the SOA between the light incident side and the light emission side makes the light confinement coefficient of the active layer on the light incident side larger than the light confinement coefficient of the active layer on the light emission side.
Since the light confinement coefficient is defined by the ratio of the cross-sectional area of the active layer to the spread of light, and the spread of light increases with the thickness of the guide layer on the light emission side where the light confinement coefficient is small, an increase in the photon density in the active layer is controlled, the gain saturation hardly occurs, and a large output light intensity is obtained.